Locating A Specific Part Of A String?

I am trying to make a program which outputs an Invoice Number, which consists of the first 3 letters of a person's first name and the last 3 digits of their zip code. The user enter their first and last name, then enters their city, state, and zip such as:

Name: Billy Jones
City, State, Zip: Chicago, Illinois 42349
Therefore, the invoice would look like:
Invoice: Bil349

I can get the first 3 letters of the first name fine using the substring function, but I have been stumped on how to get past all of the text to get the last 3 digits of the zip code entered.